Switching of spare repeater sections



Patented Jan. 21, 194i UNITED STATES SVVITCHING OF SPARE REPEATERSECTIONS Ira G. Wilson, New York, N. Y., assigner to Bell TelephoneLaboratories, k Incorporated, New York, N. Y., a corporation of New YorkApplication September 28, 1939, Serial No. 296,893

12 Claims.

'Ihis invention relates vparticularly to broadband transmission systemssuch as multichannel carrier current', signaling systems.

Although the invention is applicable to openwire circuits, wire cableand coaxial cablev circuits, it relates to problems which becomeespecially signicant in circuits carrying a large number of signalchannels, such as a coaxial cable. In such circuits it is customary touse a large number of line repeaters in tandem spaced at intervals whichmay be as short as ve miles, or even less, with several unattendedrepeaters between attended repeater points. The section between suchattended points may be perhaps fifty or one hundred miles long. Ifcertain types of failure or a degradation in the operation of any one ofthe large number of elements in the circuit occurs, such as a tubefailure, it means that the whole circuit fails or is degraded Vand thismay involve several hundred signal channels in the circuit. Thus, incer-tain coaxial systems it is proposed to transmit as many as 400 or500 channels on a single coaxial conductor. While with great precautionsthe likelihood of failures or degradation may be Very small the numberof channels involved renders it of the greater importance to makesuitable provision in the event that they do occur.

In my invention I provide certain spare facil'- ities and the purpose ofthe invention is to arrange for the simultaneous use of the sparefacilities with the regular facilities or for the substitution of suchspare facilities in place of the regular facilities to such extent as isneeded in case of failure.' Another purpose is to make any substitutionin such manner as to reduce to a minimum any interruption or anyirregularity in the transmission of the various signals or types ofsignals which are being transmitted.

The invention will be better understood by reference to the followingspecification and the accompanying drawing in which:

Fig. 1 shows an extended transmission circuit made up of severalsections and installed in duplicate for simultaneous use;

Fig. 2 is a modification of the circuit of Fig. l;

Fig. 3 shows a detail of a portion of the circuit of Fig. 2;

Fig. 4 is a modication of Fig. 2;

Fig. 5 shows a detail of a portion of Fig. 4; and

Figs. 6, 7 and 8 lare further modifications of Fig. 4.

Referring more specifically to Fig. 1 there is shown a transmissioncircuit in duplicate. While in this figure, as well as the subsequentones, the

invention will be described in terms of a coaxial conductor, it is to beundertsood that this jis for illustrative purposes only and that theinvention applies equally well to other transmission circuits such asthe ordinary cable circuits and is of par- 5 ticular use in any circuittransmittingl a broad band of signals such as is required in televisionor multichannel signaling.

In many such circuits, especially in coaxial systems, it is necessary tohave repeaters at fre- 10 quent intervals along the transmission lineand it is expedient that -certain of these ,repeaters shall beunattended, the general control of these repeaters being from certainattended repeater points. In this description it will be convenient 15to refer to a portion of the line extending from one attended point tothe next attended point as a section. In Fig. l there are shown severalsuch sections comprising a portion of an extended transmission circuit.AIn View of the possibility of 20 failure of some sort at any point in asection and in View of the large number of signal channels involved inany one circuit I provide two parallel and identical circuits for eachsection, it being evident that the likelihood of both sections fail- 25ing at the same time is much less than Ithat for one. Where Fig. 1implies that within one section the two circuits will be running inclose juxtaposition, andthis would very probably be the4 case in commonpractice, itis evident that the 30 two circuits may run by differentroutesv thus reducing still further the likelihood of both cir- -cuitsfailing at the same time. In the ligure, one circuit in each section isidentied by the referencerletter N signifying the normal circuit. The 35duplicate circuits are indicated by the letter S. These symbols,however, in this particular iigure have no special signicancefor, invaccordance with my invention, the two circuits in any ones'ection areyboth normally in operation in 40 parallel in identically the samemanner. In each section there are a plurality of spaced repeatersindicated bythe triangles, the pointing of the triangles indicating thedirection of transmission, which is shown asthe same in all the flg- 415ures. At such points as A1, B1, etc. there will be an attended repeaterand it will be convenient tobring the two circuits of a section togetherat these points, as is shown in passing from section A to thepreferablyshort section of cable at the 50 point Ar. Having passedthrough this repeater, the' output is impressed on the parallel circuitsof section B'. If in any one of the sections a failure occurs in one ofthe circuits, the signal is then carried entirely by the other circuitwith no great 55 effect or reaction on the preceding or followingsections of line.

Fig. 2 is a circuit essentially the same as Fig. 1 in which, forsimplicity, the equivalent of the 5 two-wire circuit of the coaxial isrepresented by a singleline.. In Fig. 1 the connection from thereceiving ends of section A to the common portion of the circuit at therepeater point A1 is made by ordinary transformers. I iind, however,that l0 it is extremely useful to provide for conjugacy between suchcircuits as N and S in any section and at both ends of the section. Thiscan be provided by means of suitable hybrid coil connections I-Ii and H2at each attended repeater point.

While a number of circuit arrangements can be used for establishing suchconjugaoy, one circuit which I find particularly simple is shown in Fig.3, this being a circuit which is well known in the communication art.The four-coil winding 5 has two of its coils connected to the terminalsof one incoming circuit. A balancing network M with an impedance equalto that of the outgoing line is associated as shown. With the secondincoming circuit connected in bridge as indicated,

circuits N and S will be conjugate to each other.

Such an arrangement of hybrid coil or its electrical equivalent will beassumed in the remaining figures. It will be apparent that with such anarrangement, ii so violent a change as a short circuit should occur in asection N, such short circuit will have no effect upon the S section andvice versa.

In addition to these changes, Fig. 2 takes oognizance of the fact thatthe phase shift for the two circuits of a given section may not beidentically the same. To provide for any desired adjustment of the phaseof the signal of one circuit as compared with that in another, Fig. 2 isprovided with phase shifting networks i in each circuit. In someinstances it may be desirable to so adjust the signals that they add upin phase at the output of the hybrid coil at the end of the section.More generally, however, I find it preferable to introduce a phasediiference of 120 degrees between the message coming over the twocircuits in a section. In that event, if the amplitude of a signal isrepresented by a then the vector sum of the two components will still bea and in case one of the circuits is completely disabled the amplitudeof the signal impressed on the hybrid coil is still equal to a. In caseof partial degradation of one of the circuits, the resultant of the twocomponents will not'differ very largely from the same value.

In accordance with the modification of my circuit as shown in Fig. 4 Ipropose to have the ends of the two circuits of a section associatedpermanently as already shown in connection with Figs. 1 and 2. In thiscase, however, one circuit only will be operating normally to supply asignal to succeeding sections and the other circuit will serve strictlyas a spare facility. To this end the spare circuit is disabled by eitheran open circuit or a short circuit at some point in its path, preferablyat the receive end as shown symbolically in Fig. 4. In this iigure aswell as in the previous ones it will be understood that in accordancewith standard practice in such transmission circuit, a pilot frequencywill be present on the circuit for continually and continuouslyregulating or adjusting the gain of the repeaters to values appropriatefor the eX- isting physical condition of the transmission circuit. It isassumed that such a pilot frequency is present on both the normal andthe spare circuits of Fig. 4 but with the arrangement as shown thereinthe spare circuit is shorted and therefore neither signal nor pilotfrequency pass out from the spare circuit to the hybrid coil. Pilot 5signal will however be transmitted over the normal circuit. Such signalwill be picked olf from the line by a filter F and, through an amplifierdetector, will then operate a relay R. As long as pilot signal arriveson the normal circuit l0 the spare circuit will be shorted (or opened).In case of failure of the normal circuit or degradation below a certainspecified point as indicated by a falling off of the pilot signal, therelay R will release thus enabling the spare cir- 15 cuit. It isevident, however, that as soon as the spare circuit is enabled pilotfrequency will then again come through over the spare to operate therelay R which will then interrupt the spare circuit. To prevent such acontingency a lock- 20 out contact k1 is provided such that when therelay R has been once released it will not then operate by itself.

Since the normal and the spare circuits are conjugate with respect toeach other, it is evi- 25 dent that one may now disconnect the normalcircuit at any desired points to make any needed repairs or adjustments.Having been restored to action a manual closing of the contact k1 willthen lead to the clisablement of the spare circuit 30 as formerly.

While the short-circuiting arrangement for the spare circuit o-f Fig. 4is shown as a make and break contact in the main transmission circuit ofthe spare, I nd that for the Very high 35 frequencies used in coaxialcables such contacts usually possess suiiicient capacity so that whenphysically open they do not introduce sufficient lossto be serviceable.On the other hand, I find a loss pad such as is shown in Fig. 5 to be 40eiective. In this Fig. 5 there are shown two series resistance and ashort-circuiting shunt in the main transmission line. When current isflowing through the relay Winding R1 a short across the line is removedand the series re- 45 sistance is Vshorted so that no loss is introducedin the transmission circuit. When no current flows through the winding,however, the short around the series resistance is removed and a directshort across the line is inserted, the whole 50 introducing a loss ofsufiicient magnitude to serve the purposes as desired. With this losspad I nd that even at frequencies as high as 2 megavcycles a loss ofdecibels is readily attained.

It will be observed that there are no series con- .55 tacts in thedirect transmission path. It is also clear from Fig. 5 how this loss padwould be introduced into the circuit of the spare of Fig. 4. v

Fig. 6 shows a modication of the circuit of 60 Fig. 4 in that thepick-off point for the pilot channel filter is placed directly on thenormal circuit in front of the hybrid coil. In this case it will beapparent that as long as pilot signal is coming over the normal circuitat normal level 65 the spare circuit will be disabled. If the pilotsignal fails and the relay R is released then the spare circuit isenabled. Furthermore,l since the spare and the normal circuits areconjugate to each other, pilot signal coming 'over the spare 70 circuitwill not enter through the filter F to operate the relay R. As soon,however, as the normal circuit is restored to correct operatingcondition, the pilot signal coming thereover will disable the sparecircuit and thus there is an auto- 75 matic restoration of normaloperating conditions.

Fig. 7 is similar to Fig. 4 except that either circuit may be disabledat will. In this case provision therefor is made by the introduction ineach circuit of some suitable switching device, here shown as similar tothat in Fig. 5 but somewhat simplied. In Fig. 'I the schematic form ofcircuit of Fig. 2 is used in part but in order to more fully illustratethe manner of connection, the one-line showing is extended into thetwo-line showing. Means for exciting the relays in Fig. 7 are omittedbut it is evident that they may be operated from any desired points andfrom any desired source such as direct current batteries or pilot signalor otherwise.

In Fig. 8 a further modification of the circuit of Fig. 2, is sho-wn,this circuit calling for but one hybrid coil. The regular and the sparesections are connected at the transmit end in the same manner asindicated in connection with the previous figures. Instead, howeverI ofbringing them together through a hybrid coil at the receive end,provisions are made for direct switching in the transmission path. Thusin Fig. 8 it will be observed that, through a suitable relay, contact isnormally made for transmission over the regular path to the hybrid coilof the next section. The relay for maintaining this connection iscontrolled by pilot signals coming over the two circuits, these pilotsignals being selected by amplier-detectors to energize two windings ofa biased relay. The relay, however, is so biased by direct current orotherwise that connection is normally made from the regular circuit tothe next section, the pilot signal over the two circuits virtuallyneutralizing each other. If, however, pilot signal fails or is degradedover the regular circuit, then pilot signal over the spare takes controland switches the relay contacts so that transmission now takes place tothe next section over the spare circuit.

What is claimed is:

l. A communication system comprising a section of broad-band signaltransmission line subject to impairment of service, a substantiallyidentical section serving as a spare, normal connections from thereceive end of the previous section to the transmit end of the regularand the spare sections for continuously activating both by signals, andmeans at the receive end of said regular and spare sections forconnecting them alternatively to activate the next section.

2. The combination of claim 1 characterized by the fact that the regularand spare sections are so associated with the previous section that theyare conjugate with respect to each other.

3. In a communication system comprising a section of broad band signaltransmission line subject to impairment of service, a substantiallyidentical section serving as a spare, means for concurrently activatingthe transmit ends of said sections with signals from the previoussection, means for concurrently activating the next section with signalsfrom the receive ends of both said regular and said spare sections, andphase adjusting means in one of the two firstmentioned sections to bringthe respective outputs of said two sections into a desired phaserelationship such that if one of said two sections is disabled thevectorially combined output will remain approximately the same.

4. In a communication system comprising a section of signaling circuitsubject to impairment of service, a substantially identical sectionserving as a spare, a six-terminal network connecting the transmit endsof the regular and spare sec-tions in conjugate relation with each otherfor normally concurrent activation by signal from the receive end of theprevious section, and another six-terminal network connecting thereceive ends of the regular and spare sections in conjugate relationwith each other and to the transmit end of the next section.

5. The combination of claim 4 characterized by the fact that there is aphase adjusting network in at least one of the circuits to bring theoutput of the two parallel sections into desired phase relationship.

6. The combination of claim 1 characterized by the fact that there aremeans for normally disabling the spare circuit but for enabling it whenthe regular circuit is impaired.

7. The combination of claim 1 characterized by means for normallydisabling the spare circuit at its receive end but for enabling it whenthe normal circuit is impaired.

8. In a communication system comprising a section of coaxial circuitsubject to impairment of service, a substantially identical sectionserving as a spare, a hybrid coil connection from the receive end of theprevious section to the transmit end of the regular and the sparesections whereby both the regular and the spare sections are normallyactivated by signal in parallel, a hybrid coil connection from thereceive ends of the regular and spare sections to the transmit end ofthe next section, a pad connection at the receive end of the sparesection and means controlled by the pilot signal in the normal circuitto control the pad whereby when pilot signal is normal the losses due tothe pad are high and when pilot signal is below normal the losses due tothe pad are low.

9. The combination of claim 4 characterized by the fact that there aremeans for normally disabling the spare circuit but for enabling it whenthe regular circuit is impaired.

10. The combination of claim 4 characterized by means for normallydisabling the spare circuit at its receive end but for enabling it whenthe normal circuit is impaired.

11. In a communication system comprising a section of coaxial circuitsubject to impairment of service, a substantially identical sectionserving as a spare, a hybrid coil connection from the receive end of theprevious section to the transmit end of the regular and the sparesections whereby both the regular rand the spare sections are normallyactivated by signal in parallel, and means for normally disabling thespare circuit but for enabling it when the regular circuit is impaired.

12. The combination of claim 4 characterized by the fact that there aremeans for normally disabling the spare circuit but for enabling it whenthe regular circuit is impaired, the said means being subject to pilotsignal.

IRA G. WILSON.

